Mention that MENU unlocks screen

Pressing MENU while in lock screen unlocks (it still asks for the schema
or code if enabled).

README.md

@@ -622,7 +622,7 @@ _<kbd>[Super]</kbd> is typically the <kbd>Windows</kbd> or <kbd>Cmd</kbd> key._
  | Click on `HOME`                             | <kbd>MOD</kbd>+<kbd>h</kbd> \| _Middle-click_
  | Click on `BACK`                             | <kbd>MOD</kbd>+<kbd>b</kbd> \| _Right-click²_
  | Click on `APP_SWITCH`                       | <kbd>MOD</kbd>+<kbd>s</kbd>
- | Click on `MENU`                             | <kbd>MOD</kbd>+<kbd>m</kbd>
+ | Click on `MENU` (unlock screen)             | <kbd>MOD</kbd>+<kbd>m</kbd>
  | Click on `VOLUME_UP`                        | <kbd>MOD</kbd>+<kbd>↑</kbd> _(up)_
  | Click on `VOLUME_DOWN`                      | <kbd>MOD</kbd>+<kbd>↓</kbd> _(down)_
  | Click on `POWER`                            | <kbd>MOD</kbd>+<kbd>p</kbd>